First of all, thanks for the swift reply. Then i'll try to answer you, excuse my english since it's not my speaking language
It couldn't be codec since i can play all the episodes i downloaded using the single episodes torrents. So far i got episodes from 7 to 12 and they all work correctly.
When i downloaded episodes from 5 to 9 using the "from 5 to 9 pack" torrent i also got the episodes 7, 8 and 9 that i already got in the "single torrent" way. I checked, the size of files is identical but still the files from the "pack" cannot be played while the same files from the "single torrents" work correctly. And i also tried a lot of programs and codecs as well: videolan, bsplayer, windows media player... and koepi's xvid (both rc2 and rc3 releaseas), ffdshow... just to name the most famous.
I am sure you've packed well the original source of the "pack" torrents and your files are correct, but then what i can think is that some of the leeching clients "put" some error in the files or that bit torrent makes mistakes when administering the downloads of multiple videos (not compressed in a unique file).
I really don't know, but the problem is real, and i really don't think it's only codec issues, for the matters i said before. I also tried 2 times to download the pack 5->9 and got the same error...
Thanks again for the attention.
Edit update:
I've got a program called File-Safe, amoang its features it does CRC32 check and effectively the CRCs of the file i got through the "pack" torrent are all incorrect, while *surprise* the same files (but gotten through their "single" torrents) have correct CRCs.